In cellular mobile radio system that currently exist, for example as per the GSM (Global System for Mobile Communications) standard, one or more network-side base stations communicate with stationary or mobile subscriber-side stations via radio interfaces. To enable larger geographical areas to be covered with a limited frequency bandwidth, these types of mobile radio systems are as a rule of cellular design.
Each radio cell in this case is assigned at least one radio channel, in which case each of the radio channels in its turn is assigned a restricted frequency range of the overall frequency bandwidth available. In this case the radio channels of adjacent radio cells are not used as a rule in a radio cell, to avoid mutual interference. Since however the radio field attenuation in this type of radio system is restricted faults or interference are triggered in each radio cell especially by the radio channels of the immediately adjacent radio cells. This particularly effects the boundary area in each case between the two adjacent radio cells, this occurring especially when the frequency band of the radio channel of the one radio cell is directly adjacent to the frequency band of the radio channel of the other adjoining radio cell. This type of interference is known as adjacent channel interference. The adjacent channel signal-to noise-ratio, that is the ratio between the signal power and the noise power of adjacent channels, significantly influences the spectral efficiency of a radio system.
In mobile radio receivers the received mobile radio signal is normally fed to a receive filter for filtering. As can be seen from FIG. 3 the effect of such a receive filter primarily depends on the spectral location and the power density S(f) of the adjacent channel interference. Thus, in the example shown in FIG. 3, even after receive filtering the lower end area 18 marked in black as well as in the upper cross-hatched end area 19 of the frequency spectrum of radio channel no. 1 adjacent channel interference occurs as a result of radio channels no. 0 or no. 2 of the adjacent radio cells. Whereas the adjacent channel interference remaining in the lower end area 18 is negligible, the adjacent channel interference remaining in the upper end area 19 is still relatively strong. By contrast to FIG. 3 real receive filters basically do not possess an infinite rate of change which increases even further the interference effect of the adjacent channels.
Receive filters can however not be dimensioned in such a way that on the one hand they suppress severe adjacent channel faults and thereby improve the bit error rate, but on the other hand, if adjacent channel faults do not occur they do not make the bit error rate worse. A receive filter which is optimized for both cases cannot be dimensioned.
A method for filtering a mobile radio signal is known from WO 00/72454 Al in which a mobile radio signal received over a mobile radio channel is filtered before being processed further, in which case the received mobile radio signal is analyzed with regard to the occurrence of adjacent channel interference and if the presence of adjacent channel interference is established in the received mobile radio signal, this is selectively filtered before further processing to suppress the adjacent channel interference. Analysis in this case is preferably undertaken in bursts with the frequency spectrum of the individual bursts being analysed. In particular the energy contained in the upper end of the frequency spectrum of a burst and the energy contained at the lower end of the frequency spectrum of a burst are determined and compared with each other so that for a defined deviation between the energies determined in this way conclusions can be drawn about the presence of adjacent channel interference. The mobile radio signal selectively filtered in this way is subsequently fed to an equalizer for further processing.
Currently a new radio interface technology is being introduced for the existing mobile radio system GSM, which will be referred to as EDGE (Enhanced Data Rates for GSM Evolution) or EGPRS (Enhanced General Packet Radio System). This new technology makes high bit rate mobile video and Internet applications possible, in which case the data throughput can currently be increased to up to 384 kbits/s per carrier. In order to achieve this, with EDGE the normal binary GMSK (Gaussian Minimum Shift Keying) used for GSM will be replaced by an eight-value PSK Modulation (PSK:Phase Shift Keying). This type of modulation is in general more sensitive than GMSK modulation to selective receive band filters. To achieve the required receive sensitivities for EDGE in accordance with the GSM specifications currently envisaged, the predominantly fixed selective receive band filtering used thus far is to be replaced by an adaptive filtering. For GMSK this is also a possible option or, the system being described in the previously mentioned WO 00/72454 A1. Also described in this document is a possible methodology for adaptive receive band filtering.
A receiver is known from U.S. Pat. No. 6,178,211 B1 in which a carrier frequency signal is received and converted into a digital intermediate frequency signal. The carrier frequency signal is fed, either before or after digitization, to an “Automatic Gain Control, AGC”. With the aid of an adaptive control system a signal level after channel filtering is detected in the intermediate frequency range and compared with a specified threshold. The threshold is selected to depend on the bandwidth of the intermediate frequency range in order to implement as fast switchover or a fast adaptation of the bandwidth with minimal transient effects.
A receiver with two intermediate frequency filters is known from U.S. Pat. No. 6,047,171. In this case one intermediate frequency filter exhibits a narrower bandwidth than the other intermediate frequency filter. A changeover switch is used to switch between the two intermediate frequency filters.
A method of filtering in a receiver is known from EP 0 722 226 A1. In this case an absolute signal strength is determined and on the basis of this the frequency response of a filter is compared in the receiver.